Instruments incorporating heated probe tips are used in several fields, with such instruments including soldering irons in electronics and jewelry applications. Heated probe tips are also utilized in the jewelry industry for wax casting and in the medical field for multiple uses including cauterization.
An inherent requirement of a heated probe tip is a means for achieving a heated tip. Typical methods of heating the tip include holding the tip in a flame or in contact with a resistive heater. The most common conventional method of heating probe tips uses a resistive heating element in thermal contact with the heating tip. This method is found in the common soldering iron.
An alternative method involves passing an electric current through the probe tip such that resistive heating occurs within the probe tip itself. An example of such a device is found in U.S. Pat. No. 4,527,560, issued Jul. 9, 1985.
Maintenance of a constant or predictable tip temperature utilizing these techniques is often difficult. Heat loss frown the probe tip requires that additional heat be added over time to maintain a constant heating tip temperature. Primary sources of heat loss from a heated tip, particularly a relatively narrow tip, are heat convection along the length of the tip and radiation to the surrounding atmosphere. Additionally, heat can flow from the tip to a housing in which the tip is mounted through conduction. Heat energy is also expended through the use of the heated probe tip to perform its intended function. For example, as a soldering iron heats solder, heat flows from the heating tip to the solder and to the material to which the solder is applied. The rate at which heat is lost frown the tip, and hence the rate at which heat must be supplied to maintain a constant temperature, is generally varied and unpredictable. Thus, applying heat to the tip at a constant rate will generally not maintain the tip at a constant temperature.
These multiple heat loss mechanisms require additional heat input to the probe tip over time. Moreover, because each of the heat loss mechanisms can result in differing rates of heat loss at different points in the probe tip, temperature differentials are often found along the probe tip.
Devices utilizing the heating techniques described above present difficulties in maintaining and monitoring the temperature of the heated tip. Devices employing an eternally applied resistive heating element typically attempt to maintain a high temperature by applying heat at some distance from the distal end of the heating tip. Heating of the tip then occurs through heat conduction along the heating tip to the distal end. The heat loss mechanisms described above affect this process, resulting in a temperature gradient along the heating tip. Further, heat energy can be conducted away from the distal end to the housing in which the heating tip is mounted, causing problems such as an uncomfortably hot grip for the user and degradation of the electrical components. Because typical resistive heating coils consist of round wires coiled around the heating tip, much of the heat radiated from the wires is not necessarily toward the tip, reducing efficiency.
In an attempt to maintain a constant tip temperature despite unpredictable heat losses from the tip and temperature gradients along the heated tip, U.S. Pat. No. 5,043,560 describes a heated probe in which the resistive heating element is encased inside the heated probe tip, and a thermocouple is placed at one end of the resistive heating element to monitor the temperature. While this reduces some of the problems associated with the various heat loss mechanisms, heat from the heated tip may still be conducted away by the probe tip housing. Additionally, the thermocouple described in the prior art measures the temperature at the interface between the resistive heating element and the electrical conductor. Because this temperature measurement occurs at some distance from the distal end of the heated tip, it may differ substantially from the temperature at the distal end of the heated tip. Variations in environmental and use conditions can result in variations in the heat loss mechanisms described earlier, making the measurement inaccurate.
Devices using current flow through the tip to supply heat can present similar problems. To reduce the effects of the heat loss mechanisms, the device described in U.S. Pat. No. 4,527,560 attempts to utilize a current density gradient to focus heating in the distal end of the probe tip. Although this works well for small tips, the technique of U.S. Pat. No. 4,527,560 is ineffective in larger tips because the current requirement to achieve sufficient current density through the tip for adequate heating can be problematic.